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Chapter 29 Lecture physics FOR SCIENTISTS AND ENGINEERS a strategic approach THIRD EDITION randall d. knight © 2013 Pearson Education, Inc. Chapter 29 Potential and Field Chapter Goal: To understand how the electric potential is connected to the electric field. © 2013 Pearson Education, Inc. Slide 29-2 Chapter 29 Preview © 2013 Pearson Education, Inc. Slide 29-3 Chapter 29 Preview © 2013 Pearson Education, Inc. Slide 29-4 Chapter 29 Preview © 2013 Pearson Education, Inc. Slide 29-5 Chapter 29 Preview © 2013 Pearson Education, Inc. Slide 29-6 Chapter 29 Preview © 2013 Pearson Education, Inc. Slide 29-7 Chapter 29 Preview © 2013 Pearson Education, Inc. Slide 29-8 Chapter 29 Reading Quiz © 2013 Pearson Education, Inc. Slide 29-9 Reading Question 29.1 What quantity is represented by the symbol ? A. Electronic potential. B. Excitation potential. C. emf. D. Electric stopping power. E. Exosphericity. © 2013 Pearson Education, Inc. Slide 29-10 Reading Question 29.1 What quantity is represented by the symbol ? A. Electronic potential. B. Excitation potential. C. emf. D. Electric stopping power. E. Exosphericity. © 2013 Pearson Education, Inc. Slide 29-11 Reading Question 29.2 What is the SI unit of capacitance? A. Capaciton. B. Faraday. C. Hertz. D. Henry. E. Exciton. © 2013 Pearson Education, Inc. Slide 29-12 Reading Question 29.2 What is the SI unit of capacitance? A. Capaciton. B. Faraday. C. Hertz. D. Henry. E. Exciton. © 2013 Pearson Education, Inc. Slide 29-13 Reading Question 29.3 The electric field A. Is always perpendicular to an equipotential surface. B. Is always tangent to an equipotential surface. C. Always bisects an equipotential surface. D. Makes an angle to an equipotential surface that depends on the amount of charge. © 2013 Pearson Education, Inc. Slide 29-14 Reading Question 29.3 The electric field A. Is always perpendicular to an equipotential surface. B. Is always tangent to an equipotential surface. C. Always bisects an equipotential surface. D. Makes an angle to an equipotential surface that depends on the amount of charge. © 2013 Pearson Education, Inc. Slide 29-15 Reading Question 29.4 This chapter investigated A. Parallel capacitors. B. Perpendicular capacitors. C. Series capacitors. D. Both A and B. E. Both A and C. © 2013 Pearson Education, Inc. Slide 29-16 Reading Question 29.4 This chapter investigated A. Parallel capacitors. B. Perpendicular capacitors. C. Series capacitors. D. Both A and B. E. Both A and C. © 2013 Pearson Education, Inc. Slide 29-17 Chapter 29 Content, Examples, and QuickCheck Questions © 2013 Pearson Education, Inc. Slide 29-18 Connecting Potential and Field The figure shows the four key ideas of force, field, potential energy, and potential. We know, from Chapters 10 and 11, that force and potential energy are closely related. The focus of this chapter is to establish a similar relationship between the electric field and the electric potential. © 2013 Pearson Education, Inc. Slide 29-19 Finding the Potential from the Electric Field The potential difference between two points in space is: where s is the position along a line from point i to point f. We can find the potential difference between two points if we know the electric field. Thus a graphical interpretation of the equation above is: © 2013 Pearson Education, Inc. Slide 29-20 Example 29.1 Finding the Potential © 2013 Pearson Education, Inc. Slide 29-21 Example 29.1 Finding the Potential © 2013 Pearson Education, Inc. Slide 29-22 QuickCheck 29.1 This is a graph of the x-component of the electric field along the x-axis. The potential is zero at the origin. What is the potential at x 1m? A. 2000 V. B. 1000 V. C. 0 V. D. 1000 V. E. 2000 V. © 2013 Pearson Education, Inc. Slide 29-23 QuickCheck 29.1 This is a graph of the x-component of the electric field along the x-axis. The potential is zero at the origin. What is the potential at x 1m? A. 2000 V. B. 1000 V. C. 0 V. D. 1000 V. E. 2000 V. © 2013 Pearson Education, Inc. V = –area under curve Slide 29-24 Tactics: Finding the Potential From the Electric Field © 2013 Pearson Education, Inc. Slide 29-25 Finding the Potential of a Point Charge © 2013 Pearson Education, Inc. Slide 29-26 Example 29.2 The Potential of a Parallel-Plate Capacitor © 2013 Pearson Education, Inc. Slide 29-27 Example 29.2 The Potential of a Parallel-Plate Capacitor © 2013 Pearson Education, Inc. Slide 29-28 Example 29.2 The Potential of a Parallel-Plate Capacitor © 2013 Pearson Education, Inc. Slide 29-29 Sources of Electric Potential A separation of charge creates an electric potential difference. Shuffling your feet on the carpet transfers electrons from the carpet to you, creating a potential difference between you and other objects in the room. This potential difference can cause sparks. © 2013 Pearson Education, Inc. Slide 29-30 Van de Graaff Generator © 2013 Pearson Education, Inc. Slide 29-31 Batteries and emf The most common source of electric potential is a battery. The figure shows the charge escalator model of a battery. Lifting positive charges to a positive terminal requires that work be done, and the chemical reactions within the battery provide the energy to do this work. © 2013 Pearson Education, Inc. Slide 29-32 Batteries and emf The potential difference between the terminals of an ideal battery is: where is the emf, which, long ago, was an abbreviation of “electromotive force.” A battery constructed to have an emf of 1.5 V creates a 1.5 V potential difference between its positive and negative terminals. © 2013 Pearson Education, Inc. Slide 29-33 QuickCheck 29.2 The charge escalator in a battery does 4.8 1019 J of work for each positive ion that it moves from the negative to the positive terminal. What is the battery’s emf? A. 9 V. B. 4.8 V. C. 3 V. D. 4.8 1019 V. E. I have no idea. © 2013 Pearson Education, Inc. Slide 29-34 QuickCheck 29.2 The charge escalator in a battery does 4.8 1019 J of work for each positive ion that it moves from the negative to the positive terminal. What is the battery’s emf? A. 9 V. B. 4.8 V. C. 3 V. . D. 4.8 1019 V. E. I have no idea. © 2013 Pearson Education, Inc. Slide 29-35 Batteries in Series The total potential difference of batteries in series is simply the sum of their individual terminal voltages: Flashlight batteries are placed in series to create twice the potential difference of one battery. For this flashlight: Vseries V1 V2 1.5 V 1.5 V 3.0 V © 2013 Pearson Education, Inc. Slide 29-36 Finding the Electric Field from the Potential The figure shows two points i and f separated by a small distance s. The work done by the electric field as a small charge q moves from i to f is W Fss qEss. The potential difference between the points is: The electric field in the s-direction is Es V/s. In the limit s 0: © 2013 Pearson Education, Inc. Slide 29-37 Finding the Electric Field from the Potential Quick Example Suppose we knew the potential of a point charge to be V q/4 0r but didn’t remember the electric field. Symmetry requires that the field point straight outward from the charge, with only a radial component Er. If we choose the s-axis to be in the radial direction, parallel to E, we find: This is, indeed, the well-known electric field of a point charge! © 2013 Pearson Education, Inc. Slide 29-38 Example 29.3 The Electric Field of a Ring of Charge © 2013 Pearson Education, Inc. Slide 29-39 Example 29.3 The Electric Field of a Ring of Charge © 2013 Pearson Education, Inc. Slide 29-40 Example 29.4 Finding E From the Slope of V © 2013 Pearson Education, Inc. Slide 29-41 Example 29.4 Finding E From the Slope of V © 2013 Pearson Education, Inc. Slide 29-42 Example 29.4 Finding E From the Slope of V © 2013 Pearson Education, Inc. Slide 29-43 QuickCheck 29.3 At which point is the electric field stronger? A. At xA. B. At xB. C. The field is the same strength at both. D. There’s not enough information to tell. © 2013 Pearson Education, Inc. Slide 29-44 QuickCheck 29.3 At which point is the electric field stronger? A. At xA. |E| = slope of potential graph B. At xB. C. The field is the same strength at both. D. There’s not enough information to tell. © 2013 Pearson Education, Inc. Slide 29-45 QuickCheck 29.4 An electron is released from rest at x 2 m in the potential shown. What does the electron do right after being released? A. Stay at x 2 m. B. Move to the right ( x) at steady speed. C. Move to the right with increasing speed. D. Move to the left (x) at steady speed. E. Move to the left with increasing speed. © 2013 Pearson Education, Inc. Slide 29-46 QuickCheck 29.4 An electron is released from rest at x 2 m in the potential shown. What does the electron do right after being released? A. Stay at x 2 m. B. Move to the right ( x) at steady speed. C. Move to the right with increasing speed. Slope of V negative => Ex is positive D. Move to the left (x) at steady speed. E. Move to the left with increasing speed. (field to the right). Electron is negative => force to the left. Force to the left => acceleration to the left. © 2013 Pearson Education, Inc. Slide 29-47 The Geometry of Potential and Field In three dimensions, we can find the electric field from the electric potential as: © 2013 Pearson Education, Inc. Slide 29-48 QuickCheck 29.5 Which set of equipotential surfaces matches this electric field? © 2013 Pearson Education, Inc. Slide 29-49 QuickCheck 29.5 Which set of equipotential surfaces matches this electric field? © 2013 Pearson Education, Inc. Slide 29-50 QuickCheck 29.6 The electric field at the dot is A. 10î V/m. B. 10î V/m. C. 20î V/m. D. 30î V/m. E. 30î V/m. © 2013 Pearson Education, Inc. Slide 29-51 QuickCheck 29.6 The electric field at the dot is A. 10î V/m. B. 10î V/m. C. 20î V/m. D. 30î V/m. 20 V over 2 m, pointing toward lower potential E. 30î V/m. © 2013 Pearson Education, Inc. Slide 29-52 Kirchhoff’s Loop Law For any path that starts and ends at the same point: The sum of all the potential differences encountered while moving around a loop or closed path is zero. This statement is known as Kirchhoff’s loop law. © 2013 Pearson Education, Inc. Slide 29-53 QuickCheck 29.7 A particle follows the trajectory shown from initial position i to final position f. The potential difference V is A. 100 V. B. 50 V. C. 0 V. D. 50 V. E. 100 V. © 2013 Pearson Education, Inc. Slide 29-54 QuickCheck 29.7 A particle follows the trajectory shown from initial position i to final position f. The potential difference ΔV is A. 100 V. B. 50 V. C. 0 V. D. 50 V. E. 100 V. © 2013 Pearson Education, Inc. V = Vfinal – Vinitial, independent of the path Slide 29-55 A Conductor in Electrostatic Equilibrium © 2013 Pearson Education, Inc. Slide 29-56 A Conductor in Electrostatic Equilibrium When a conductor is in equilibrium: All excess charge sits on the surface. The surface is an equipotential. The electric field inside is zero. The external electric field is perpendicular to the surface at the surface. The electric field is strongest at sharp corners of the conductor’s surface. © 2013 Pearson Education, Inc. A corona discharge, with crackling noises and glimmers of light, occurs at pointed metal tips where the electric field can be very strong. Slide 29-57 A Conductor in Electrostatic Equilibrium The figure shows a negatively charged metal sphere near a flat metal plate. Since a conductor surface must be an equipotential, the equipotential surfaces close to each electrode roughly match the shape of the electrode. © 2013 Pearson Education, Inc. Slide 29-58 QuickCheck 29.8 Metal wires are attached to the terminals of a 3 V battery. What is the potential difference between points 1 and 2? A. 6 V. B. 3 V. C. 0 V. D. Undefined. E. Not enough information to tell. © 2013 Pearson Education, Inc. Slide 29-59 QuickCheck 29.8 Metal wires are attached to the terminals of a 3 V battery. What is the potential difference between points 1 and 2? Every point on this conductor is at the same potential as the positive terminal of the battery. A. 6 V. B. 3 V. C. 0 V. D. Undefined. E. Not enough information to tell. © 2013 Pearson Education, Inc. Every point on this conductor is at the same potential as the negative terminal of the battery. Slide 29-60 QuickCheck 29.9 Metal spheres 1 and 2 are connected by a metal wire. What quantities do spheres 1 and 2 have in common? A. Same potential. B. Same electric field. C. Same charge. D. Both A and B. E. Both A and C. © 2013 Pearson Education, Inc. Slide 29-61 QuickCheck 29.9 Metal spheres 1 and 2 are connected by a metal wire. What quantities do spheres 1 and 2 have in common? A. Same potential. B. Same electric field. C. Same charge. D. Both A and B. E. Both A and C. © 2013 Pearson Education, Inc. Slide 29-62 Capacitance and Capacitors The figure shows a capacitor just after it has been connected to a battery. Current will flow in this manner for a nanosecond or so until the capacitor is fully charged. © 2013 Pearson Education, Inc. Slide 29-63 Capacitance and Capacitors The figure shows a fully charged capacitor. Now the system is in electrostatic equilibrium. Capacitance always refers to the charge per voltage on a fully charged capacitor. © 2013 Pearson Education, Inc. Slide 29-64 Capacitance and Capacitors The ratio of the charge Q to the potential difference VC is called the capacitance C: Capacitance is a purely geometric property of two electrodes because it depends only on their surface area and spacing. The SI unit of capacitance is the farad: The charge on the capacitor plates is directly proportional to the potential difference between the plates: © 2013 Pearson Education, Inc. Slide 29-65 QuickCheck 29.10 What is the capacitance of these two electrodes? A. 8 nF. B. 4 nF. C. 2 nF. D. 1 nF. E. Some other value. © 2013 Pearson Education, Inc. Slide 29-66 QuickCheck 29.10 What is the capacitance of these two electrodes? A. 8 nF. B. 4 nF. C. 2 nF. D. 1 nF. E. Some other value. © 2013 Pearson Education, Inc. Slide 29-67 Capacitance and Capacitors Capacitors are important elements in electric circuits. They come in a variety of sizes and shapes. © 2013 Pearson Education, Inc. The keys on most computer keyboards are capacitor switches. Pressing the key pushes two capacitor plates closer together, increasing their capacitance. Slide 29-68 Example 29.6 Charging a Capacitor © 2013 Pearson Education, Inc. Slide 29-69 Forming a Capacitor The figure shows two arbitrary electrodes charged to Q. It might appear that the capacitance depends on the amount of charge, but the potential difference is proportional to Q. Consequently, the capacitance depends only on the geometry of the electrodes. © 2013 Pearson Education, Inc. Slide 29-70 Combinations of Capacitors In practice, two or more capacitors are sometimes joined together. The circuit diagrams below illustrate two basic combinations: parallel capacitors and series capacitors. © 2013 Pearson Education, Inc. Slide 29-71 Capacitors Combined in Parallel Consider two capacitors C1 and C2 connected in parallel. The total charge drawn from the battery is Q = Q1 + Q2. In figure (b) we have replaced the capacitors with a single “equivalent” capacitor: Ceq = C1 + C2 © 2013 Pearson Education, Inc. Slide 29-72 Capacitors Combined in Parallel If capacitors C1, C2, C3, … are in parallel, their equivalent capacitance is: © 2013 Pearson Education, Inc. Slide 29-73 QuickCheck 29.11 The equivalent capacitance is A. 9 F. B. 6 F. C. 3 F. D. 2 F. E. 1 F. © 2013 Pearson Education, Inc. Slide 29-74 QuickCheck 29.11 The equivalent capacitance is A. 9 F. B. 6 F. C. 3 F. D. 2 F. E. 1 F. © 2013 Pearson Education, Inc. Parallel => add Slide 29-75 Capacitors Combined in Series Consider two capacitors C1 and C2 connected in series. The total potential difference across both capacitors is VC = V1 + V2. The inverse of the equivalent capacitance is: © 2013 Pearson Education, Inc. Slide 29-76 Capacitors Combined in Series If capacitors C1, C2, C3, … are in series, their equivalent capacitance is: © 2013 Pearson Education, Inc. Slide 29-77 QuickCheck 29.12 The equivalent capacitance is A. 9 F. B. 6 F. C. 3 F. D. 2 F. E. 1 F. © 2013 Pearson Education, Inc. Slide 29-78 QuickCheck 29.12 The equivalent capacitance is A. 9 F. B. 6 F. C. 3 F. D. 2 F. Series => inverse of sum of inverses E. 1 F. © 2013 Pearson Education, Inc. Slide 29-79 The Energy Stored in a Capacitor The figure shows a capacitor being charged. As a small charge dq is lifted to a higher potential, the potential energy of the capacitor increases by: The total energy transferred from the battery to the capacitor is: © 2013 Pearson Education, Inc. Slide 29-80 The Energy Stored in a Capacitor Capacitors are important elements in electric circuits because of their ability to store energy. The charge on the two plates is q and this charge separation establishes a potential difference V q/C between the two electrodes. In terms of the capacitor’s potential difference, the potential energy stored in a capacitor is: © 2013 Pearson Education, Inc. Slide 29-81 The Energy Stored in a Capacitor A capacitor can be charged slowly but then can release the energy very quickly. An important medical application of capacitors is the defibrillator. A heart attack or a serious injury can cause the heart to enter a state known as fibrillation in which the heart muscles twitch randomly and cannot pump blood. A strong electric shock through the chest completely stops the heart, giving the cells that control the heart’s rhythm a chance to restore the proper heartbeat. © 2013 Pearson Education, Inc. Slide 29-82 QuickCheck 29.13 A capacitor charged to 1.5 V stores 2.0 mJ of energy. If the capacitor is charged to 3.0 V, it will store A. 1.0 mJ. B. 2.0 mJ. C. 4.0 mJ. D. 6.0 mJ. E. 8.0 mJ. © 2013 Pearson Education, Inc. Slide 29-83 QuickCheck 29.13 A capacitor charged to 1.5 V stores 2.0 mJ of energy. If the capacitor is charged to 3.0 V, it will store A. 1.0 mJ. B. 2.0 mJ. C. 4.0 mJ. D. 6.0 mJ. E. 8.0 mJ. © 2013 Pearson Education, Inc. UC (V)2 Slide 29-84 Example 29.8 Storing Energy in a Capacitor © 2013 Pearson Education, Inc. Slide 29-85 Example 29.8 Storing Energy in a Capacitor © 2013 Pearson Education, Inc. Slide 29-86 The Energy in the Electric Field The energy density of an electric field, such as the one inside a capacitor, is: The energy density has units J/m3. © 2013 Pearson Education, Inc. Slide 29-87 Dielectrics The figure shows a parallel-plate capacitor with the plates separated by a vacuum. When the capacitor is fully charged to voltage (VC)0, the charge on the plates will be Q0, where Q0 C0(VC)0. In this section the subscript 0 refers to a vacuum-filled capacitor. © 2013 Pearson Education, Inc. Slide 29-88 Dielectrics Now an insulating material is slipped between the capacitor plates. An insulator in an electric field is called a dielectric. The charge on the capacitor plates does not change (Q = Q0). However, the voltage has decreased: VC < (VC)0 © 2013 Pearson Education, Inc. Slide 29-89 Dielectrics The figure shows how an insulating material becomes polarized in an external electric field. The insulator as a whole is still neutral, but the external electric field separates positive and negative charge. © 2013 Pearson Education, Inc. Slide 29-90 Dielectrics © 2013 Pearson Education, Inc. Slide 29-91 Dielectrics © 2013 Pearson Education, Inc. Slide 29-92 Dielectrics We define the dielectric constant: The dielectric constant, like density or specific heat, is a property of a material. Easily polarized materials have larger dielectric constants than materials not easily polarized. Vacuum has = 1 exactly. Filling a capacitor with a dielectric increases the capacitance by a factor equal to the dielectric constant: © 2013 Pearson Education, Inc. Slide 29-93 Dielectrics The production of a practical capacitor, as shown, almost always involves the use of a solid or liquid dielectric. All materials have a maximum electric field they can sustain without breakdown—the production of a spark. The breakdown electric field of air is about 3 106 V/m. A material’s maximum sustainable electric field is called its dielectric strength. © 2013 Pearson Education, Inc. Slide 29-94 Dielectrics © 2013 Pearson Education, Inc. Slide 29-95 Example 29.9 A Water-Filled Capacitor © 2013 Pearson Education, Inc. Slide 29-96 Example 29.9 A Water-Filled Capacitor © 2013 Pearson Education, Inc. Slide 29-97 Example 29.9 A Water-Filled Capacitor © 2013 Pearson Education, Inc. Slide 29-98 Example 29.9 A Water-Filled Capacitor © 2013 Pearson Education, Inc. Slide 29-99 Example 29.10 Energy Density of a Defibrillator © 2013 Pearson Education, Inc. Slide 29-100 Example 29.10 Energy Density of a Defibrillator © 2013 Pearson Education, Inc. Slide 29-101 Example 29.10 Energy Density of a Defibrillator © 2013 Pearson Education, Inc. Slide 29-102 Example 29.10 Energy Density of a Defibrillator © 2013 Pearson Education, Inc. Slide 29-103 Chapter 29 Summary Slides © 2013 Pearson Education, Inc. Slide 29-104 General Principles © 2013 Pearson Education, Inc. Slide 29-105 General Principles © 2013 Pearson Education, Inc. Slide 29-106 General Principles © 2013 Pearson Education, Inc. Slide 29-107 Important Concepts © 2013 Pearson Education, Inc. Slide 29-108 Important Concepts © 2013 Pearson Education, Inc. Slide 29-109